CVE-2026-40912
Published: 30 April 2026
Summary
CVE-2026-40912 is a high-severity Use of Incorrectly-Resolved Name or Reference (CWE-706) vulnerability in Traefik Traefik. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 49.0% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).
Deeper analysis
CVE-2026-40912 is a high-severity authentication bypass vulnerability (CVSS 3.1 score of 8.2) affecting Traefik, an HTTP reverse proxy and load balancer. The issue resides in the StripPrefixRegex middleware when combined with ForwardAuth, BasicAuth, or DigestAuth, impacting versions prior to 2.11.43, 3.6.14, and 3.7.0-rc.2. Specifically, the middleware applies regex matching against the decoded URL path but uses the resulting byte length to slice the percent-encoded raw path. If a dot (or multiple dots) appears in the URL prefix, the stripped raw path becomes a dot-segment (e.g., /./admin/secret) per RFC 3986, which evades protection mechanisms.
An unauthenticated network attacker (AV:N/AC:L/PR:N) can exploit this by crafting a request with dots in the prefix portion of a protected URL path. The dot-segment path is forwarded in the X-Forwarded-Uri header to ForwardAuth, which fails to match it against protected patterns, allowing the request to proceed. The backend then normalizes the dot-segment to the actual protected path, resulting in unauthorized access to sensitive content (C:H/I:L/A:N). This works against any backend performing dot-segment normalization as defined in RFC 3986.
Traefik has addressed the vulnerability in patched releases: version 2.11.43, 3.6.14, and 3.7.0-rc.2. Security practitioners should upgrade immediately to these versions, as detailed in the official GitHub release notes and security advisory (GHSA-6jwx-7vp4-9847). The issue is linked to CWE-706 (Use of Incorrectly-Resolved Name Into a Reference Chain).
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-26428
Vulnerability details
Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's StripPrefixRegex middleware when used in combination with ForwardAuth, BasicAuth, or DigestAuth. The middleware matches…
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the regex against the decoded URL path but uses the resulting byte length to slice the percent-encoded raw path. When a dot (or multiple dots) appears in the prefix portion of the URL, the raw path after stripping becomes a dot-segment (e.g. /./admin/secret). ForwardAuth receives this dot-segment path in X-Forwarded-Uri, which does not match the protected path patterns and therefore allows the request through. The backend then normalizes the dot-segment to the real path per RFC 3986 and serves the protected content An unauthenticated attacker can exploit this against any backend that performs dot-segment normalization. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The vulnerability is an authentication bypass in the public-facing Traefik reverse proxy/load balancer, directly enabling remote exploitation of a public-facing application by unauthenticated attackers crafting malicious requests to access protected resources.
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Requires timely identification, reporting, and remediation of software flaws like the StripPrefixRegex authentication bypass in vulnerable Traefik versions, preventing exploitation via patching.
Mandates validation of URL path inputs in the reverse proxy to detect and reject dot-segment evasions (e.g., /./admin/secret) before forwarding to authentication middlewares.
Controls communications at system boundaries through properly configured proxies or WAFs that enforce consistent URL path normalization and matching to block crafted requests evading ForwardAuth.